The document EP-A-0.231.513 describes a device for obtaining gaseous mixtures, particularly anesthetic, comprised by a vector gas in which is injected a measured quantity of an additive. The device comprises a closed chamber, maintained at constant temperature and pressure, this pressure and this temperature being such that the additive is in gaseous phase. An outlet passage connects the enclosure to a conduit into which is sent the vector gas. This passage is provided with measuring means constituted by a valve which is cyclicly opened and/or a valve constituting an adjustable constriction. The enclosure is connected to a closed receptacle containing the additive in liquid phase, such that, each time the additive leaves the enclosure through the outlet passage, an equal mass of liquid additive enters the enclosure to be vaporized therein. To do this, it suffices that the liquid level in the receptacle be lower than the enclosure, that the connection between the receptacle and the enclosure be provided by a tube which dips into the liquid, and that the pressure above the liquid be defined as a function of the pressure which it is desired to maintain in the enclosure.
In the device described in the cited document, the regulation of the additive flow rate through the outlet passage, which is to say finally the additive content of the mixture, is carried out by acting on the frequency of opening of the valve in the outlet passage.
This operative mode has the drawback that it has no intrinsic safety: if the adjustment valve becomes blocked in open position, or closed position, in the course of anesthesia, the consequences can be immediately serious.
It will also be noted that the pressure in the enclosure must always be substantially less than that which corresponds to the boiling of the liquid at the temperature in this enclosure. Thus, if the pressure in the enclosure were to mount beyond this limit value, the dosage means would begin to add the additive in liquid phase and no longer in gaseous phase. There would result a very abrupt increase, and possibly very dangerous, of the additive content of the mixture. Such an accident could take place as a result of stopping the heating means of the enclosure. As the operation is at constant pressure, it would thereby be necessary that the safety means block the measuring means before the liquid additive can reach the latter. In this case, stopping heating would give rise to an abrupt fall in the additive content of the mixture.
The apparatus must therefore be associated with alarm and safety devices which respond very rapidly, which increase its complication and cost.
Another drawback of the device described in the mentioned document results from the fact that the adjustment of the additive flow rate is made by acting on the frequency of opening the measuring valve. For small contents, the frequency could fall to 0.1 Hz, namely one pulse every ten seconds. Such low frequencies can require the presence, downstream, of a mixing means, which is a further complication.